1. Field of the Invention
The present invention relates to a power amplifier and more particularly to a power amplifier for audio apparatuses having an improved power efficiency and distortion characteristic, which can be manufactured at low costs.
2. Prior Art
There has been proposed a power amplifier which realizes high power efficiency and low distortion by combining an amplifier circuit having a low distortion factor and an amplifier circuit having a high power efficiency. In the construction of one such conventional power amplifier, a neutral point between positive and negative power supply sources for powering an amplifier to amplify an input signal thereof in a Class-A mode is driven by an output of another amplifier which functions to amplify the above-mentioned input signal in a Class-B mode. More specifically, as shown in FIG. 1, a first amplifier 1 amplifies an input signal, applied to an input terminal 2, in a Class-A mode and delivers an amplified input signal to a loudspeaker 3, the amplifier 1 having a pair of positive and negative voltage input terminals connected respectively to serially-connected floating voltage sources 4a and 4b each having an output voltage V.sub.L. The voltage V.sub.L is set to the minimum level required for the operation of the amplifier 1. A second amplifier 5 amplifies the input signal in a Class-B mode and has a pair of positive and negative voltage input terminals supplied respectively with output voltages +V.sub.H and -V.sub.H of fixed voltage sources 6a and 6b (V.sub.H &gt;V.sub.L). With this construction, the output of the second amplifier 5 drives the junction point of the floating voltage sources 4a and 4b. Thus, the voltage sources 4a and 4b produce the output voltage set to the minimum level required for operating the first amplifier 1 in a Class-A mode, and the junction point of the voltage sources 4a and 4b is varied in accordance with the input signal. With this arrangement, the conventional power amplifier of FIG. 1 achieves improved distortion and dynamic range. However, this power amplifier has the following disadvantages:
(a) In the case where the floating voltage sources 4a and 4b are of such a construction that ac voltage is rectified to produce an output voltage, common-mode noises may appear on the output voltage of this power amplifier due to stray capacitance of coils of the associated transformer.
(b) In the case where the power amplifier is used in a stereo sound system, the floating voltage sources 4a and 4b must be provided in each of right and left channels of the stereo sound system. This increases the manufacturing cost of the system.
(c) When the input signal of a great amplitude is applied to the first amplifier 1, the output voltage increases to a level almost equal to the output voltages +V.sub.H and -V.sub.H of the voltage sources 6a and 6b. Therefore, such voltages +V.sub.H and -V.sub.H must be supplied to those stages of the amplifier 1 up to its drive stage, so that these stages must be composed of circuit elements resistant to a high voltage. This also increases the manufacturing cost of the power amplifier.